The aim of this project is to determine the influence of specific structural features of the 5'-terminal portion of eukaryotic mRNA on initiation of protein synthesis in vitro. By comparing the ribosome-binding efficiencies of mRNA's containing different 5' ends the following conclusions have been reached: 5'-terminal m7G has a strong promoting influence on mRNA selection by eukaryotic ribosomes. Among the different 5'-termini, m7GpppAm appears to be enriched in ribosome-associated mRNA, indicating a greater affinity of this cap to ribosomes or initiation factors. Similarly the presence of 2'-O-methyl groups promotes the binding of mRNA to ribosomes. N6-methylation of A caps increases efficiency of initiation complex formation only at high potassium ion concentrations.